The assembly of the present invention is particularly well suited for certain types of strapping tools wherein a tensioning means, such as a motor driven tension wheel, is brought into contact with one of the overlapping strap ends of the strap loop and rotated to pull that strap end to constrict the loop about the package. Such tools typically draw tension in the loop to a predetermined or preset level and then subsequently crimp a seal about the overlapping strap ends and sever the trailing portion of the strap. Examples of such tools are described in the U.S. Pat. No. 3,506,041 to Angarola, and the U.S. Pat. No. 3,198,218 to Ericsson et al.
With tools of the type described above, mechanisms have been devised to automatically sense the tension level in the loop and to terminate the tensioning when that level is reached. In the tool described in the aforementioned Angarola patent, the tensioning action is terminated when a predetermined resistance torque in the strap tensioning wheel is encountered. This corresponds approximately to the desired final loop tension level. Specifically, the supply pressure to an air motor is controlled such that, at the desired tension, the supply pressure can no longer rotate the output shaft of the motor to turn the feed wheel against the loop tension and the motor "stalls." Further, in the tool described in the Angarola patent the subsequent strap severing and seal-crimping actions are effected in response to the sensing of a predetermined back-up pressure in the air supply line leading to the tensioning motor when the air motor stalls or approaches stall.
Although the tool described in the aforementioned Angarola patent functions well under a variety of conditions with many types of strap, it would be desirable to provide a more direct control of the termination of the tensioning sequence and of the initiation of the strap severing and seal-crimping sequences rather than to rely upon sensing of air pressures in the tool.
It would be desirable to provide a strapping tool having an air motor actuatable by depressing a lever which opens a valve to supply air to the air motor. It would be advantageous if the valve could be subsequently automatically latched in this open position so that the operator can then release the lever while the strap loop is automatically tensioned. After the preset tension level has been reached, the valve would preferably be automatically unlatched to terminate the tensioning. However, where low elongation strap is used and where the strap is pulled relatively tightly around an article by hand before initiating the tensioning cycle, it is possible that the operator might not be able to release the lever quickly enough to allow the valve to open upon automatic release of the hold down latch. In this situation the air motor would then tension the loop beyond the desired preset tension level. Thus, it would be desirable to provide an actuation system that would permit the tension motor to be automatically stopped when the preset tension level is reached even if the operator is still depressing the actuating lever.